1. Field of the Invention
The present invention relates to a method for driving a plasma display panel, and more particularly, to a method for driving a color plasma display panel capable of tonal display by dividing one field into a plurality of sub-fields to set the number of times of emission for each sub-field to different values.
2. Description of the Related Art
Conventionally, the tonal display on a plasma display panel has been implemented by controlling a number of times of discharge (emission luminance) during a maintenance period as shown in FIG. 11. More specifically, one field (F), which displays one screen, is repeated 50 to about 70 times a second, whereby screens of the respective fields are stacked by means of afterimages of a human eye and a flicker-free natural image can be obtained. This one-field period is divided into a plurality of sub-fields (SF), and these sub-fields are combined by varying a maintenance pulse number (a number of times of discharge) during the maintenance period of each sub-field to thereby implement tonal display.
In, for example, display of 64 shades of gray, as shown in FIG. 11, one field is constituted by six sub-fields: SF1 to SF6, and a preliminary discharge period (preliminary lighting period+blanking period) is provided at the head of each sub-field, and subsequent to this period, there are provided a write period and a maintenance period respectively. The weighting is effected by reducing the number of times of discharge during these maintenance periods by about 1/2 for each successive sub-field, from the sub-field at the head (in SF1, the number of times of discharge is assumed to be 32n where n is a positive integer).
When the foregoing sub-field is selected within one frame for maintenance discharge in accordance with this method, the emission luminance can be controlled by the number of times of maintenance discharge in the sub-field selected, and therefore, the display of 64 shades of gray can be implemented.
In this respect, FIG. 12 is a sectional view showing a general plasma display panel. In FIG. 12, reference numeral 1 designates a front substrate; 2a, a scanning electrode; 2b, a maintenance electrode; 3, a bus electrode; 4, a dielectric layer; 5, a rear substrate; 6, a data electrode; 7, a white dielectric body; 8, fluorescent material; and 9, a discharge cell respectively.
If the preliminary discharge periods are provided at the heads of all the sub-fields as described above, preliminary discharge occurs at least six times even in a non-display portion to cause light emission over the entire screen. This emission causes black float particularly in a dark place, thereby deteriorating the contrast. Also, if the sub-fields are arranged simply in decreasing order of the weighting of emission luminance (number of times of discharge) as shown in FIG. 11, a pseudo contour may appear on displaying a moving image.
In order to suppress these defects, a driving sequence, as shown in FIG. 1, is used (this driving sequence diagram in FIG. 1 is the same as that for the present invention), in which this preliminary discharge is applied once per field, and the sub-fields are not arranged simply in decreasing order of the weighting of emission luminance (number of times of discharge), but their sequence has been determined by contriving. In such driving sequence, the preliminary discharge period is provided only for the sub-field SF6 at the head, and the sub-field SF6 is constituted by the preliminary discharge period, a write period, a maintenance period and a maintenance blanking period. Each of the sub-fields SF1 to 5 other than the sub-field SF6 is constituted by a write period, a maintenance period and a maintenance blanking period.
In such driving sequence in which preliminary discharge is provided for all sub-fields as shown in FIG. 11, the sequence, in which light is certainly emitted over the entire screen at the beginning of each sub-field for blanking, is adopted, and therefore, the presence or absence of wall charge, which is caused by the presence or absence of maintenance discharge of the sub-field in question, is bound to be erased, and does not affect the next sub-field. In contrast, however, in such driving sequence, in which the preliminary discharge is thinned out, as shown in FIG. 1, the presence or absence of the maintenance discharge during a maintenance period of the sub-field in question remains as a difference in wall charge on the scanning electrode and maintenance electrode, and therefore, the blanking characteristics of a maintenance blanking period provided at the last of the sub-field becomes important as one of the elements for determining the operating margin.
However, wall charge has conventionally been blanked by the use of microdischarge using wall charge during the maintenance blanking period, and therefore, the maintenance blanking period is susceptible to the amount of wall charge, and the blanking characteristics easily becomes unstable. Therefore, when it is adopted, such sub-field driving sequence as shown in FIG. 1 is defective, in that the operating margin is lowered and the yield is reduced as compared with the conventional method in which all sub-fields are provided with preliminary discharge.
FIG. 13 shows the dependence of the operating margin in driving sequence in sub-fields of FIG. 1 on the sub-field. The "minimum operating voltage" in this figure is the minimum value of the drivable voltage, and the "maximum operating voltage" is the maximum value of the drivable voltage. This operable voltage range is the operating margin. When voltage exceeding this operating margin is applied, an erroneous display occurs, and when voltage below the operating margin is applied, a non-display portion occurs. From this figure, it can be seen that the operating margin of the sub-field next to a sub-field having low weighting of emission luminance is lowered.
In other words, SF4, which is next to SF6 having the minimum emission luminance, has the highest minimum operating voltage, and the lowest maximum operating voltage. From this figure, therefore, it can also be seen that the operating margin for the entire plasma display panel is regulated by SF4 to be narrowed. The sub-field SF4, which is next to SF6 having the minimum emission luminance, has the minimum operating margin. This is because the intensity of the maintenance discharge during a maintenance period prior to the maintenance blanking period is affected by the maintenance pulse number constituting the maintenance period.
As shown in FIG. 14, the maintenance discharge during the maintenance period becomes stronger with the number of maintenance pulses PSUS to be applied, and will be saturated. Therefore, when the number of maintenance pulses is as small as 1 piece (case of n=1) like SF6, the maintenance discharge does not become strong during the maintenance period. On the other hand, at SF3, which follows SF1, the maintenance discharge becomes strong because the number of maintenance pulses at SF1 is as sufficiently great large as 32 pieces (case of n=1).
Since the number of the maintenance pulses differs depending on the sub-field as described above, the intensity of the maintenance discharge differs, and the amounts of wall charge which are produced by the respective sub-fields during the maintenance period are different from one another. Since these different wall charge have been blanked (neutralized) during the maintenance blanking period having the same maintenance blanking pulse, the blanking (neutralization) of the wall charge becomes insufficient in a sub-field having a small number of maintenance pulses, leading to decrease in the foregoing operating margin.
In this respect, a driving method in which the preliminary discharge is not provided for all the sub-fields, but the number of times of preliminary discharge per field is reduced in an attempt to enhance the display contrast, is discussed in Japanese Patent Application Laid-Open Nos. 4-280289 and 7-49663. Also, a conventional example in which the waveform of the blanking pulse has been contrived in order to obtain sufficient blanking characteristics even if there are variations in the characteristics of the discharge cell, is discussed in Japanese Patent Application Laid-Open Nos. 8-30228 and 9-160522. They are aimed to eliminate variations in the blanking characteristics within one field and discharge cell.